The cardiovascular effects of intraventricular 5,6-dihydroxytryptamine in conscious hypertensive rats.

1. Conscious experimental hypertensive rats injected intraventricularly with 5,6-dihydroxytryptamine (50 microgram) responded with a rapid and long-lasting (4 days) fall in blood pressure and this was accompanied by a bradycardia of similar duration. 2. The acute actions of 5,6-dihydroxytryptamine on the cardiovascular system were not modified by pretreatment with bromolysergide (0.5 mg/kg, i.p.), methysergide (2.5 mg/kg, i.p.) or intraventricular phentolamine (200 microgram). 3. The results suggest that central serotonergic neurones play an important role in the regulation of blood pressure. However, the site of action remains to be investigated.